This invention relates to a battery pack used as the power source for electric power tools.
A battery pack used for an electric power tool is often used under extremely harsh environmental conditions. For example, when used with a highly loaded motor, instantaneous load currents become exceedingly large and batteries discharge large currents over short time intervals. In addition, the battery pack may be used in an environment with significantly high ambient temperature. Finally, re-use of the battery pack can be required as soon as possible after battery discharge. Therefore, batteries may be rapidly charged at high currents and used in a high temperature state.
A battery pack housing a circuit breaker has been developed to protect batteries from this type of environment. The circuit breaker is connected in series with the batteries and cuts-off current when battery temperature becomes abnormally high or when battery current becomes abnormally large. For this reason, the circuit breaker contains a system to detect battery temperature and cut-off current, and a system to detect current flow in the batteries and cut-off current.
A battery pack housing a circuit breaker cuts-off current and protects the batteries, for example, when the motor of an electric power tool locks-up and extremely large current near that of a short circuit flows. Further, current is cut-off to protect the batteries when continuous high current results in an abnormal rise in battery temperature.
A battery pack which houses a circuit breaker has the feature that batteries can be protected to extend their lifetime. However, since the circuit breaker cuts-off current using a device such as a thermostat to sense temperature, when the temperature drops, the circuit breaker automatically resets. When the circuit breaker resets, current again flows and a motor of the electric power tool may begin turning possibly leading to a dangerous situation. Particularly in an electric power tool with a power switch which can be maintained in the on position, if a motor locks-up and the circuit breaker cuts-off current, motor rotation will stop without turning the power switch off. In this case, one may forget to turn the power switch off. A dangerous situation can develop if a circuit breaker resets to turn over a motor in an electric power tool with its power switch in the on position. Of particular danger is sudden rotation of an electric power tool motor which has cutting blades attached.
The present invention was developed to solve this problem. Thus it is a primary object of this invention to provide a battery pack for electric power tools which not only protects batteries with a circuit breaker, but is also extremely safe to use.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.